Using Radiosondes as cheap GPS trackers

A Radiosonde is an inexpensive sensor package that’s intended to be used with a weather balloon for atmospheric measurements. The device transmits data in the 403 MHz band after being launched and they usually aren’t recovered after use. You can pick them up for very little money on eBay so [Nick] thought they might work well as a low cost GPS tracker. Unfortunately the Radiosonde doesn’t transmit standard NMEA GPS data, but GPS doppler measurements. It’s hard to determine what those are actually useful for. Nick did find one other paper documenting an unsuccessful attempt which he has posted to his site. So now Nick is looking for some help either making the data more useful or coming up with a functional device that’s just as inexpensive. Anyone got any leads?

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21 thoughts on “Using Radiosondes as cheap GPS trackers”

I had bought several Vaisala RS80 Radiosondes off E-bay six months ago or so, and was also unsucessful at figuring them out. I created a Yahoo Forum for Vaisala RS80 Radiosondes (at http://groups.yahoo.com/group/Vaisala_RS80_Radiosonde/ ) and received this response, but never found the time to follow up on it; it may be helpful…

Good Luck!
Ken_S.
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Hi

Members of this group might be interested in the SondeMonitor group (also on Yahoo). SondeMonitor is a program that can decode the data from Vaisala sondes and display the measurements in various ways.

The RS80 transmits Pressure, Temperature and Humidity using analogue tones in the range 8-10kHz on a 400MHz carrier. Provided that you have a suitable radio receiver, SondeMonitor can measure and plot these tones so that you can watch the sonde as it goes up (and down again).

The more modern RS92 sondes are digital and in addition to the meteorological measurements, it also sends GPS data down in the telemetry. SondeMonitor can plot the met data from these digital sondes but it can also plot the ground track from the GPS data. This opens the possibility of tracking and perhaps catching one of these digital sondes.

You can find formulas on the web for calculating location from the doppler signals. The math is fairly hairy, though. On the other hand, that sondemonitor program, for one, will do the math for you. There’s bound to be some other GPS calculating software out there.

If the sonde is the one mentioned from Sippican, then right there on the data sheet most of the information needed is presented to fetch the data. Now, calculations.. another story.
He has the radio reception, but he mentioned hoping there would be a chip that would decode the audio.. well.. not so much a chip as an OLD MODEM. Certainly some hacking would be needed on the modem, but 400 baud digital biphase over an audio circuit is about as simple as it gets for an old telephone modem! The audio from the receiver should couple straight in? All my knowledge of radio systems is theory and no practice so I could be way off here.

And.. if all he wants is the GPS itself, then that talks to the micro that puts everything out over radio by RS232 (again, stated right there in the datasheet).

This may not be all that helpful, however, if someone does manage to figure out how to interpret the signal, perhaps with a computer, I’d recommend conscidering an fpga for the actual processing.

Fpgas can do things in parralel, making up for their mediocre speed. Decryption, analysis, perhaps involving FFT on a binary stream (you have to have an A-D converter) are all very parralizable. You can have the fpga manage stuff like position smoothing and deltas through doplar and such.

My understanding is that the CDMA cell phones don’t have full GPS receivers in them, either. They simply transmit the doppler measurements to the network, and the NETWORK does all the hard work of figuring out what location a doppler reading corresponds to.

This is good because the network is a lot more powerful than your phone, so can get a location from a single reading, instead of taking a number of minutes to get an initial fix — an advantage for a battery-limited device.

So anyway, cell phone GPS might be a good avenue to explore after all…

Ok, since I do not have that device (the first I have heard of) and the protocol it uses, following will be purely theoratical (exactly what I do for living!):

You can use convolution theorem to obtain particular speed relative to the sender sattelite if you can overlap the incoming signals at different times, if you have exactly the differentiation at a specific interval, the process is even easier. Convolotion programs are cheap cpu-wise and source code can be found for example in numerical recipes in c. Then what you need to do is have a lock on at least 3 different sattelites and triangulate your coordinates using simple vector texel geometry. This is also very cheap. If you want error correction
then you can connect one or more sattelites depending on your algorithm. If you do not want complicated electronics, all these steps can be done using analog components (ever heard of analog computers?)

The problem is decoding of the signal I guess.
Is there a possiblity that the outcoming signal is analog or some ADC data?

I have some of these sondes. The GPS is send as a satelite number + AFAIK a 24 bit velocity, not that difficult to see on a protocol analyser. So I guess to use the signal, one uses a stationary reference receiver and an ephemeris, to see where the various satelites are. Then integrate the speed to find the position. The LORAN sondes seem to modulate the raw LORAN signal on the 403 MHz carrier (what I saw on an oscilloscope) I picked up 24 of these on eBay for about

Possible application – tracking ground movement relative to a stationary benchmark. This could be useful to scientists remotely tracking glacial movement, seismic displacement near volcanoes and fault lines, etc. The shift in coordinates could be recorded at a central unit on the benchmark, and remotely phoned in once a day. I know nothing about what systems are already out there, or how to put the whole thing together – I bow to your collective expertise. It seemed like a good idea.